xtensa_modules variable defined in each xtensa-modules.c.inc is only
used locally by the including file. Make it static.

Reported-by: Yury Gribov <tetra2005@gmail.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>